1. Field of the Invention
This invention relates to distance measuring devices, particularly of the kind which obtain distance information to an object by measuring light reflected from the object.
2. Description of Related Art
Known active type distance measuring devices include a light projecting element and a light receiving photo-sensitive element spaced a predetermined distance from each other along a base line. They are operated by turning either the light projecting element or light receiving element until the output of the light receiving photo-sensitive element reaches a peak value. Focusing a camera's photo-taking lens with such a device involves interclocking movement of the relevant element with the movement of the lens, and stopping the movement of the lens when the output of the light receiving photo-sensitive element reaches the peak value. Devices of this kind require a mechanical arrangement to turn either the light projecting element or the light receiving element. Furthermore, the distance measurements are ordinarily performed concurrently with the focusing movement of the photo-taking lens, and the latter accompanies the camera's shutter release operation. Hence, cameras using such devices cannot offer distance information before a shutter release operation. In order to measure a distance to an object within a view finder field prior to a shutter release operation, the camera must be supplemented with some additional equipment. A known automatic focus control system which is capable of such prefocusing is the double-image coincidence type. However, such a system not only uses an expensive charge coupled element but also requires a complex processing and computing circuit which is unsuited for a compact camera.
Japanese Patent Application Laid-Open No. SHO 57-44809 discloses a method which attempts to solve this problem. Here, a light projecting element is used with a semi-conductor position detector and a distance signal is obtained from a difference signal (A-B) representing the difference between two outputs A and B at different points on the semi-conductor position detector. To accurately correlate the difference signal (A-B) with the distance signal, the output of the light projecting element in this Japanese document is feedback controlled with pulse modulation signals applied to the light projecting element by a sum signal (A+B) representing the sum of the two outputs A and b, so that the sum signal (A+B) can be kept constant irrespective of the object distance. This arrangement not only requires complex feedback control circuitry but also involves the possibility of trouble with regard to oscillations. Another known arrangement involves computing a quotient (A-B) / (A+B) to cancel the dependency on the reflection factor of the object by normalizing the difference signal (A-B) with the sum signal (A+B). This necessitates use of a relatively complex division circuit and is incapable of adequately meeting todays requirements that call for reduction in the scale of circuitry of compact cameras.
Normalizing the difference signal (A-B) with the sum signal (A+B) is important in an automatic focus control system using two light receiving photo-sensitive elements in order to measure distances without introducing the effect of the reflection factor of the object to be photographed. To accomplish this and yet avoid the aformentioned deficiencies, Japanese Patent Application No. SHO 58-127409 filed by the assignee of the present application proposes normalization with a double integrating system using a Miller integration circuit (Miller integrator). However, such normalization integrates noise mixed into the distance measurment. Therefore, while uneven truncation decreases, only the output of one of two light receiving photo-sensitive elements is received as a distance measurement signal. Applying this system to a distance measuring device with a semi-conductor position detector, hardly produces satisfactory results in terms of the limit and accuracy of distance measurement.
Moreover many distance measuring devices do not have any means for detecting an object at infinity. Conceivably, one can supplement a distance measuring device with an infinity determining circuit, such as the one disclosed in the aforementioned Japanese Patent Application No. SHO 58-127409 (a signal A integrating arrangement). However, such a device has been found inadequate for use with a system based on the values of A-B.